Methodology for evaluating DP crane-barge operation in close proximity of a FPSO

This paper presents an analysis methodology for evaluating the operation of a DP crane-barge in close proximity to a FPSO. Offshore crane operations are critical, since the barge is kept in close proximity with other vessel or platform, and large loads are transported in a pendulum configuration. A precise positioning of the crane-barge is required, in order to avoid unsafe relative motions, as well as keep the load being transported on a stable position. The operation analyzed in the present paper, as an example, is the replacement of a process equipment from the plant of a turret moored FPSO. Several aspects are previously assessed by means of numerical simulation in order to guarantee a safe operation, such as: relative motions between both vessels, relative motion of the FPSO and the equipment in pendulum configuration, tension in the lifting cables, barge DP performance and station keeping capacity. A full nonlinear time domain simulator (Numerical Offshore Tank – TPN) is used to perform the coupled analysis of the system subjected to several environmental conditions, considering also the dynamics of the suspended load. The hydrodynamic interference between the FPSO and the barge are considered by means of potential calculation of wave effects. CFD model is used to a preliminary assessment of the current shadow zone induced by the FPSO and the effects on the barge DP power requirement. The simulator also considers the DP follow-target mode that is used during the operation, in which the barge follows the FPSO guaranteeing a safe distance between them.